Do not use the "-o" option of the acp_commander. It will not work. Do not use "-addons" either, because it will copy incompatible software onto the box.

Is this guide working for the LS-VL only?

No, this should work for all current LS with current firmware (>= 1.36 and maybe even the ones before).
According to the firmware readme of the LS series, the following models all should work the same way:

What will be done?

We will use the "-c" feature of acp_commander. This will execute the command as user root (without a profile loaded).

We will also use the share "share" to copy some files for gaining access.

We will not use telnet but ssh for "opening".

What is necessary?

You need a private/public key pair for ssh.
You can create such a pair using Putty on Windows. The private key will be used by Putty and the public key will be transfered to the LS and used by sshd.
On Linux you can create the keypair with ssh_keygen.

The guide

Please replace the IP address within the commandlines below against the one from your LS.

Put your public key in a file named authorized_keys and put the file onto the share "share" (do not use a subdirectory).

First we will create the directory for the ssh keys which the box should accept for logins as root.